How many trains per hour can fit into one downtown tunnel, both currently and with tunnel upgrades?
Sound Transit’s current ceiling is 20 trains per hour, or one every 3 minutes. With capital upgrades this could be increased to 30-45 trains per hour, or one every 2 to 1.5 minutes, as many other metro tunnels around the world operate. 30 trains/hour is common in many subways like New York, London, Moscow, and St Petersburg. The Paris Metro gets up to 40 trains/hour. Automated Skytrain can reach 45 trains/hour.
The 1 Line currently runs every 8 minutes peak, or 8 trains/hour. The 2 Line will have the same, or combined 4-minute frequency and 16 trains/hour. Sound Transit is currently upgrading the signals in the tunnel to ensure it can deliver that reliably; that will require two weekend closures in January. That leaves a theoretical 4 slots unused to fit within the 20/hour limit, equivalent to a line every 15 minutes.
With three lines in the tunnel all at 8-minute frequency, that’s 24 trains per hour, or a train every 2.5 minutes. That’s 4 trains over the 20/hour limit.
If those three lines are each increased to 6-minute frequency, that’s 10 trains per hour each, or a total of 30 trains/hour. That would combine for one train every 2 minutes. That’s at the low end of the upgrade range, so the easiest to do. It’s also what the tunnel alternatives study seems to be targeting to meet ridership demand in the 2030s and 2040s with more Link lines to more areas.
It’s not that Link can’t go over the 3-minute limit now. It currently runs trains every 1.5 minutes after ballgames, with extra trains in the Stadium-Roosevelt segment, some reversing on both tracks, and southbound trains from Lynnwood coming in between whenever they can. But this level of service throws reliability out the window, so trains come whenever they can. And it wouldn’t be allowed for every day service due to the limited number of egress paths in the downtown tunnel stations currently (elevators/ escalators/stairs).
There are two sets of future tunnel upgrades. One is needed anyway regardless of whether the second tunnel is built, to bring reliability up peer cities’ norm. The other is to increase tunnel capacity if we put three lines in the tunnel or make Ballard a Ballard-Westlake stub line instead of building the second tunnel. Both of these lists of projects are still being identified, but we know both of them involve signal work, and the second involves adding egress paths to Westlake station and maybe others. Other kinds of upgrades are probably needed too, but ST is still identifying them so we don’t know what they are yet.
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